Allow Shimmie to work with older versions of FFmpeg

This fixes issue #466

The problem was that older versions of FFmpeg cannot use the "scale" filter to correct for aspect ratio when generating a thumbnail. Previously the code was written assuming that it was running with the newer versions of FFmpeg and could use this feature.

This now adds an option to turn this aspect ratio correction on/off -- with the default being off.

This allows Shimmie to work "out of the box" with older versions of FFmpeg in order to generate the thumbnails, although they might be slightly distorted. The admin can change this setting latter once they have a newer version of FFmpeg.